Symposium CT04—Predictive Synthesis and Decisive Characterization of Emerging Quantum Materials

Using the example of multiferroic bismuth ferrite, we demonstrate how the interaction between the spontaneous ferroelectric polarization and the half-quantum-containing polarization lattice have a profound influence on the surface properties. With the help of density functional calculations, we identify energetically favorable happy (100) surface geometries, which are combinations of surface termination and polarization direction that lead to uncharged, stable surfaces. Switching the polarization causes these (100) surfaces considerable electrostatic distress, which must be compensated by the introduction of charged point defects or adsorbates such as water. We predict that the relative happiness or distress of the oppositely polarized surfaces should lead to an effective water splitting cycle on the bismuth ferrite (100) surface through polarization switching. We close by showing that there is an analogy between surface charge arising from ferroelectric polarization, and surface magnetization arising from magnetoelectric multipolization.

This work is in collaboration with Chiara Gattinoni and Ipek Efe

Point defects have played a major role in tuning the properties of materials over the last few decades. Controlling individual point defects in quantum heterostructures based on complex oxides presents new challenges, arising mostly from non-stoichiometry inherent to oxides. Here, we demonstrate the ability to tune point defects in LaAlO₃/SrTiO₃ (LAO/STO) oxide-based quantum heterostructures, using a newly-developed metal-organic pulsed laser deposition (MOPLD) growth technique with Ti flux provided by titanium tetraisopropoxide (TTIP). X-ray diffraction and Raman spectroscopy show that this approach opens a wide process window in which stoichiometry of STO can be controlled without structural changes. Depth-resolved cathodoluminescence spectroscopy reveals that STO films grown at higher TTIP flux have larger ratios of antisite Ti/Sr vacancies with lower concentration of oxygen vacancies. This leads to higher two-dimensional electron gas low-temperature mobilities at the LAO/STO interface and clear Shubnikov–de Haas oscillations. This result provides an essential part of the development of the next-generation complex oxide thin films and their heterostructures to investigate novel quantum phenomena.

This work was supported by the US Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, under Award DE-FG02-06ER46327.

Electronic correlation is arguably the most profound notion in condensed matter physics and often arises due to the strong interactions between the lattice and the electron charge, spin and orbital. Disentangling these degrees of freedom is thus critical to our understanding and manipulation of materials’ properties but remains challenging due to the intricate interplay between these players. The difficulty fundamentally originates from the complicated energy landscape formed from often competing order parameters with almost degenerate energies. Here we demonstrate how a handful of experimental methods – electron diffraction, transport and X-ray pair distribution function – conspire to depict the role of atomic lattice vs. the electrons in the formation of electronic correlations.

In the Mott insulator Sr₂IrO₄ which structurally and electronically resembling the canonical high T_C superconductor cuprates, we recently observed a nematic order in the nonmagnetic normal state of the parent compound persisting below the Neel temperature and into the doped samples. The newly discovered nematic order has a correlation only a few nanometers and couples strongly to the lattice through the iridium 5d orbitals. We suspect the nematic order comes from competing ordering instabilities through the ‘hidden’ Fermi surface. Our observation challenges the accepted notions of nematicity in quantum materials and suggests that electronic nematicity can exist even in a correlated insulator and may cast new light on the understanding of non-conventional superconductivity.

In the past decade, researchers have uncovered a rich electronic phase diagram between the Mott insulating antiferromagnet Sr₂IrO₄ and the superconductor Sr₂RuO₄. However, sample size has constrained available measurements, and sample quality may be obscuring emergent magnetic phases. Here we describe the synthesis of single crystalline Sr₂Ir_xRu_1-xO₄ (0<x≤0.6) via a floating zone melting technique. We find that the use of a high pressure gas environment (∼100 atm mixed O₂ and Ar) greatly decreases the evaporation of the IrO₂ reactant. The resultant gram-sized samples are more uniform in chemical composition and demonstrate unique magnetotransport properties compared to previous work on flux-grown samples.

Precise control of defects is paramount for control and manipulation of quantum phenomena. Defect formation at complex oxide interfaces can be caused by extreme environments. The inability to predict and control defect creation and evolution in environments, such as those encountered during catalytic cycling, predicates the ability to control and manipulate quantum properties and behavior and to access unique phenomena. To gain such control, it is important to understand the structural and chemical signatures associated with different defect types. LaFeO3 (LFO) is a novel perovskite system with strongly defect-regulated catalytic properties, which makes it a rich system to explore defect manipulation. This work focuses on characterizing cation and oxygen defect populations developed near interfaces in single-crystal LFO grown on (001)-oriented SrTiO3 substrates before and after electrocatalytic cycling. Scanning transmission electron microscopy (STEM) analyses are conducted using multiple modes of high-angle annular dark field, bright field, and annular bright field imaging to visualize the material microstructure. To complement imaging, local chemical and composition mapping is performed using electron energy loss spectroscopy (STEM-EELS). Our STEM-EELS measurements focus primarily on the O K edge, which is highly sensitive to local bonding and oxygen coordination environment. We discuss observed changes and their potential impact on the performance of this material.

The coexistence of two order parameters is a particular occurrence in bulk single-phase materials. Such materials possessing (anti)ferromagnetism, ferroelectricity, and ferroelasticity are known as multiferroics. BeCr₂O₄ belonging to Pbnm and Pcmn space group is one of the earliest reported multiferroic compounds. In this work, we revisit this mostly unexplored material with a focus on multiple magnetic transitions at low temperatures. In this compound, Cr (III) occupies octahedral 4a site and Be (II) occupies the tetrahedral 4c site. This adopts a close packing structure with a 90° and 138° Cr – O – Cr bonds allowing for interesting magnetic superexchange interactions. In the present work, BeCr₂O₄ powder has been prepared using solid-state reaction method and verified the phase purity and crystal structure using laboratory and synchrotron X-ray diffraction. The Rietveld refinement of the powder X-ray data was carried out using Pbnm orthorhombic space group yielding lattice parameters, a = 4.556 Å, b = 9.791 Å, c = 5.664 Å. The magnetic susceptibility as a function of temperature revealed two close-by anomalies at T_N1 ~ 26.5 K and T_N2 ~ 25.2 K, along with a third transition around 7.5 K, which are also observed in specific heat. Using the Curie-Weiss fit in the paramagnetic region, an effective magnetic moment is estimated to be 2.4 μ_B/Cr and the Curie-Weiss temperature is estimated to be -38 K indicating antiferromagnetic interactions. The reported antiferromagnetic structure is a noncollinear cycloidal spiral with a propagation vector along the c-axis with a large periodicity of 65 Å, and abnormally low Cr moment, 1.55 µ_B. We will present microscopic details of multiple magnetic orders which are previously unreported, using magnetometry, specific heat, synchrotron X-ray diffraction, and neutron diffraction data.

Investigating the dispersion of phonons and collective charge modes with high surface sensitivity is best achieved with high-resolution electron energy loss spectroscopy (HREELS) [1]. However, a systematic dispersion measurement over the whole Brillouin zone can takes weeks with standard instruments, which measure the electron intensity sequentially, i.e., at one specific loss energy and one scattering angle at a time. Therefore, we have modified a high-resolution electron source to meet the requirements of commercial hemispherical electron analyzers with E(k) imaging capabilities [2]. This allows the parallel detection of electrons in a broad range of momenta without sample movement. In this contribution I will show application examples of this new development for phonon, plasmon and exciton dispersion on 2D materials, quantum materials and organic single crystals.

[1] H. Ibach and D. L. Mills, Electron energy loss spectroscopy and surface vibrations,
Academic Press, 1982.
[2] H. Ibach, F. C. Bocquet, J. Sforzini, S. Soubatch, and F. S. Tautz, Electron energy loss spectroscopy with parallel readout of energy and momentum,
Rev. Sci. Instrum. 88, 033903 (2017).

The performance of the neutron source and instruments at the Spallation Neutron Source have now reached most of their full potential. The improvements in efficiency for inelastic neutron scattering are factors of hundreds beyond what was available a decade ago. With new experimental techniques, the momentum transfer Q can be aligned along all directions in a crystal, measuring energy spectra of phonons or magnons at all points in the Brillouin zone. Thermodynamic quantities that are sums over all phonons in a crystal can now be assessed phonon-by-phonon, and as functions of temperature.

Inelastic neutron scattering was used to measure all individual phonons in a single crystal of NaBr at temperatures of 10, 300 and 700 K. Even at 300 K the phonons, especially the optical phonons, showed large shifts in energy, with large broadenings from anharmonicity. Thermal expansion can be calculated by minimizing the free energy with respect to volume, V. In simple cases the free energy depends only on elastic energy and phonon entropy. Calculations were first performed with the quasiharmonic approximation (QHA), in which the phonon frequencies depend only on V, and on T only insofar as it alters V by thermal expansion. This QHA was an unqualified failure for predicting the temperature dependence of phonon energies, even 300 K. Ab initio computations of free energy that included anharmonicity from phonon-phonon interactions (and quasiharmonicity) successfully predicted both the temperature dependence of phonons and the large thermal expansion of NaBr. The anharmonicity was shown to arise the cubic anharmonicity of nearest-neighbor Na-Br bonds. Anharmonicity is not a small correction to the QHA predictions of thermal expansion and thermal phonon shifts, but dominates the behavior.

A quantum Langevin model, similar to models developed for optomechanics, was used to predict intermodulation phonon sidebands (IPS). Ab initio calculations of anharmonic phonons in rocksalt NaBr showed sideband features qualitatively as "many-body effects." Inelastic neutron scattering measurements on a crystal of NaBr revealed diffuse intensity at high phonon energy from the upper sideband. Its partner, the lower sideband, proves to be an "intrinsic localized mode." The spectral weight in the IPS pair is broadened and redistributed by interaction with the thermal bath, treated in the Langevin model as random noise.

These new studies on NaBr were made possible by new experimental capabilities for inelastic neutron scattering. The study of thermal expansion required the temperature dependence of all phonons in the first Brillouin zone. The observation of IPS required low backgrounds and high intensities. Both these studies are now practical with inelastic neutron scattering.

- D.S. Kim, et al., "Nuclear quantum effect with pure anharmonicity and the anomalous thermal expansion of silicon," P.N.A.S115, 1992 (2018).
- Y. Shen, et al., "The Anharmonic Origin of the Large Thermal Expansion of NaBr," Phys. Rev. Lett. 125, 085504 (2020). DOI: 10.1103/PhysRevLett.125.085504.

Dynamic control over phases of matter with electromagnetic (EM) radiation receives increasing popularity in recent years because of both its ultrafast time scale and its access to thermodynamically unapproachable phenomena. Till now, the realization of dynamic manipulation of unconventional orders, such as a high-rank multipolar order, awaits to be explored largely because the coupling between multipolar orders and EM fields is nonlinear and nontrivial. In this talk, using the commensurate charge density wave (CCDW) in 1T-TaS₂ as the archetype, we demonstrate the dynamic control over the ferro-rotational order, the antisymmetric components of the second-rank electric quadrupolar order. We first show the ferro-rotational nature of CCDW in 1T-TaS₂, broken mirrors but preserved inversion, by performing temperature-dependent rotation anisotropy-second harmonic generation (RA-SHG). We then present the dynamic modulation of this ferro-rotational CCDW order, using time-resolved-RA-SHG (tr-RA-SHG) that adopts the optical-pump, RA-SHG-probe scheme. We show that this ultrafast modulation manifests itself as the breathing and the rotation of RA-SHG patterns at three different frequencies in the neighborhood of the previously reported CCDW amplitude mode frequency, with the mode of the highest (lowest) frequency primarily in the breathing (rotation) channel and the middle one in both channels. We further reveal a sudden shift of these three frequencies and a dramatic increase in the breathing and rotation magnitudes across a critical pump fluence of ~ 0.5 mJ/cm², by performing fluence dependent tr-RA-SHG.

Quantum materials under excitation from ultrafast laser pulses have shown exotic properties, such as ultrafast topological phase transition [1] and formation of transient charge density wave [2]. A complete understanding of such dynamical phenomena requires multimodal characterization techniques to reveal the dynamics associated with spin, charge, orbital and lattice degrees of freedom under nonequilibrium states. Ultrafast electron diffraction (UED) is an emerging tool to respond to this requirement by its capability of resolving transient lattice dynamics. SLAC National Accelerator Laboratory has built a UED with mega-electron-volt (MeV) electron beams to push the instrumental resolution to atomic spatial and temporal scales [3,4]. In this talk, the instrumental performance of SLAC MeV UED as well as the configuration of the experimental platform will be briefly reviewed. Selected experimental results on topological materials, charge density wave materials as well as heterostructures from two-dimensional materials will be highlighted. Research and development efforts to further expand the capabilities of SLAC MeV UED will be discussed.
References:
[1] E. J. Sie, et al., Nature 565, 61 (2019)
[2] A. Kogar, et al., Nat. Phys. 16, 159 (2019)
[3] S. P. Weathersby, et al., Rev. Sci. Instrum. 86, 73702 (2015)
[4] X. Shen, et al., Ultramicroscopy 184(Pt A), 172 (2017)

Over the last decade, III-V semiconductor nanowires (NWs) have significantly attracted researchers due to one-dimensional architecture, quantum confinement effects, and a higher tolerance for stress-strain mismatch that allow greater freedom in engineering combinations of material systems in a variety of NW architectures to meet the demands of next-generation optoelectronic devices. Dopant incorporation in a well-controlled manner is essential to produce abrupt interfaces, to engineer work functions, and to realize advanced devices in NW configuration successfully. Unfortunately, the knowledge obtained from the thin film studies on dopant incorporation and carrier concentration cannot be directly translated to NWs due to the dopant’s influence on the growth kinetics and growth mechanisms. Hall effect, field-effect, and capacitance-voltage are the commonly used measurement techniques in thin films for the determination of carrier concentration that require highly sophisticated lithography steps. For the assessment of dopants in NWs, several characterization methods have evolved. However, the sample preparation of off-axis electron holography is complex and requires additional information about the NW and homogeneity. Secondary ion mass spectrometry and atom probe tomography require a standard of known dopant concentration and are destructive. Hence, conductive-atomic force microscopy (C-AFM), scanning Kelvin probe microscopy (SKPM), X-ray photoelectron spectroscopy (XPS), and ultraviolet photoelectron spectroscopy (UPS), provide a set of excellent characterization methods for doping assessment, as they do not involve any complex contact fabrications.
In this work, we evaluate the incorporation of Beryllium (Be) dopants in GaAs NWs and Tellurium (Te) dopants in GaAsSb NWs grown using self-catalyzed molecular beam epitaxy (MBE) with variation in Be cell temperatures (T_Be) from 750 ^oC - 950 ^oC and Te cell temperatures (T_GaTe) from 500 ^oC - 570 ^oC, respectively. The extensive research interest in GaAs and GaAsSb NWs stems from its narrow bandgap region covering the important optical telecommunication wavelengths of 1.3 µm and 1.55 µm.
The topographical and I-V measurements using C-AFM of the vertical single NW exhibited a significant enhancement in current with photoresponse of 4 nA and 5 nA at -1 V for GaAs doped with T_Be at 950 ^oC and GaAsSb doped with T_GaTe at 550 ^oC. A statistical increase in the carrier concentration of Be-doped GaAs at T_Be of 750 ^oC, 850 ^oC, and 950 ^oC is found to be 9 x 10¹⁵ cm^-3, 7 x 10¹⁷ cm^-3, and 5 x 10¹⁸ cm^-3, respectively from COMSOL Multiphysics fitting. An estimated carrier concentration of 7.1 x 10¹⁷ cm^-3, 3.0 x 10¹⁹ cm^-3, and 3.0 x 10¹⁸ cm^-3 are determined for the Te–doped GaAsSb NWs at T_GaTe of 500 ^oC, 550 ^oC, and 570 ^oC, respectively. The elemental composition, work function, and carrier concentration of Be-doped GaAs and Te-doped GaAsSb NWs are further determined using XPS/UPS. With increasing T_Be for GaAs NWs, a shift in Fermi level (FL) towards the valence band provided evidence towards higher Be incorporation with T_Be-950 ^oC having work function and carrier concentration of 5.6 eV and 6.0 x 10¹⁸ cm^-3. In Te–doped GaAsSb NWs, the FL is shifted towards the conduction band. The work function and the carrier concentration of Te – doped GaAsSb NWs at T_GaTe of 550 ^oC are found to be 3.6 eV and 2.8 x 10¹⁹ cm^-3, respectively. The values of electron density from UPS concur very well with the values determined from C-AFM. The change in surface potential observed in doped NWs in SKPM also provided strong evidence of Be and Te incorporation. Hence, these surface analytical tools are found to be powerful characterization techniques for direct measurement of the dopant levels in NWs, which are critical for bandgap engineering design of optoelectronic devices.
Acknowledgment
This material is based upon research supported by the Air Force Office of Scientific Research (AFOSR) under grant number W911NF1910002.

Over the last 100 years, the group-theoretic characterization of crystalline solids has provided the foundational language for diverse problems in physics and chemistry. There exist two classes of crystalline solids: nonmagnetic crystals left invariant by space groups (SGs), and solids with commensurate magnetic order that respect the symmetries of magnetic space groups (MSGs). Whereas many of the properties of the SGs, such as their momentum-space corepresentations (coreps) and elementary band coreps (EBRs) were tabulated with relative ease, progress on deriving the analogous properties of the MSGs has largely stalled for the past 70 years due to the complicated symmetries of magnetic crystals. In this work, we complete the 100-year-old problem of crystalline group theory by deriving the small coreps, momentum stars, compatibility relations, and magnetic EBRs (MEBRs) of the single (spinless) and double (spinful) MSGs. We have implemented freely-accessible tools on the Bilbao Crystallographic Server for accessing the coreps of the MSGs, whose wide-ranging applications include neutron diffraction investigations of magnetic structure, the interplay of lattice regularization and (symmetry-enhanced) fermion doubling, and magnetic topological phases, such as axion insulators and spin liquids. Using the MEBRs, we extend the earlier theory of Topological Quantum Chemistry to the MSGs to form a complete, real-space theory of band topology in magnetic and nonmagnetic crystalline solids - Magnetic Topological Quantum Chemistry (MTQC). We then use this theory to perform a high through-put search of topological materials search, uncovering hundreds of topological magnetic materials.

Gapped surface states are a prerequisite for realizing several desired topological states like the quantum anomalous Hall effect showing dissipationless chiral edge states, the topological axion states displaying quantized magnetoelectric effects, and Majorana fermions obeying non-Abelian statistics. Recent research interest has turned extensively toward MnBi₂Te₄, an intrinsic magnetic topological insulator, which is predicted to possess a large exchange gap in the surface states, potentially to exhibit exotic effects at room temperature. However, spectroscopic measurements with angle-resolved photoemission spectroscopy (ARPES) have shown contradictory results on the key feature needed for novel topological behavior: a magnetically induced gap between electronic energy bands. Some groups have observed the surface gap, while the others found no gap at Dirac point. The controversy can be ascribed to the large bulk carrier density of MnBi₂Te₄ coming from its highly electron-doped nature, whose fluctuation can significantly broaden the dispersion in ARPES data and overwhelm the fine structures around the Dirac point. It is thus imperative to engineer the materials to reduce the bulk carriers and to probe the surface electronic structures using local spectroscopy like scanning tunneling microscopy/spectroscopy (STM/STS).
Here we realize gapped surface states by tuning the bulk carrier density with Sb-substitution in MnBi_2-xSb_xTe₄ (MBST) and characterizing the local band structure with quasiparticle interference (QPI) STM. Minimizing bulk carrier density results in the bulk-insulating MBST and allows us to access the surface states. A surface band gap of 50 meV has been revealed around the Dirac point inside the bulk band gap. In situ transport spectroscopy using our unique multiprobe STM has confirmed the surface nature of the carriers at the Fermi level through the exhibition of 100 % surface-dominant conductance. Moreover, the surface band gap is found to be topologically protected and robust against magnetic field up to 9 T. Theoretical calculations based on density functional theory (DFT) have corroborated very well the experimental results, which reveal a topological axion insulator behavior of the MBST, consistent with recent observation of QAH effect.
This research was performed at the Center for Nanophase Materials Sciences which is a DOE Office of Science User Facility.

Magnetic topological material provides a great platform for discovering new topological states, such as the axion insulators, the Chern insulators, and the 3D quantum anomalous Hall (QAH) insulators. Recently, MnBi2Te4 was discovered to be the first material realization of a van der Waals intrinsic antiferromagnetic topological insulator (TI) where the QAH effect was observed at a record high temperature in its two-dimensional limit. Since the interplay of the magnetism and band topology determines their topological natures, understanding and manipulating the magnetism inside magnetic TIs will be crucial. In this talk, I will present our discovery of two new magnetic topological materials MnBi2Te4.nBi2Te3 (n=1 and 3) which consist of alternating [MnBi2Te4] and n[Bi2Te3] layers [1, 2]. I will show that by reducing the interlayer magnetic coupling with the increasing number of spacer [Bi2Te3] layers, MnBi2Te4.nBi2Te3 can be tuned from Z2 antiferromagnetic TIs (n=0,1,2) to ferromagnetic axion insulators. Furthermore, I will show that a continuous fine control of the magnetism in MnBi4Te7 can be made by Sb doping where an AFM to FM switching emerges due to the formation of the Mn/Sb antisite disorders [3]. Our study provides a rare tunable material platform to investigate various emergent phenomena arising from the marriage of magnetism and band topology.
[1] C. W. Hu, et.al, Nature Communications, 11, 97 (2020)
[2] C. W. Hu, et.al, Science Advances, 6, eaba4275 (2020)
[3] C. W. Hu, et.al, ArXiv: 2008.09097 (2020)

The spin Hall effect (SHE) has its special position in the field of spintronics, which allows transforming a charge current into a spin current and vice versa without the use of magnetic materials or magnetic fields. To gain new insight into the physics of the SHE and to identify materials with a substantial spin Hall conductivity (SHC), with the developed automatic high-symmetric atomic Wannier function projection, we performed high precision, high-throughput ab initio electronic structure calculations of the intrinsic SHC for over 20,000 non-magnetic crystals. The calculations reveal a strong and unexpected relation of the magnitude of the SHC with the crystalline symmetry, which we show exists because large SHC is typically associated with mirror symmetry protected nodal lines in the band structure. From the newly developed database, we identify new promising materials. This includes materials with a SHC comparable or even larger than that the up to now record Pt as well as materials with different types of spin currents. Furthermore, we find that the existence and magnitude of unusual symmetry spin currents that do not have perpendicular directions of spin current flow, spin polarization, and the electrical field can be in many materials tuned by the orientation of the electrical field with respect to the crystal [1]. This could be helpful for designing new types of spin-orbitronics devices.
[1] arXiv:1909.09605

Magnetic skyrmions, which are 3-dimensional topological excitations with a nano-sized particle character, have come to the forefront as potential applications in spintronic devices. These vortex-like spin nanostructures are stabilized by Dzyaloshinskii-Moriya asymmetric exchange interaction facilitated by strong spin-orbit coupling and broken inversion symmetry crystal lattice. However, designing new skyrmion-host materials presents a significant challenge. To address this, we propose a chemical strategy for inducing skyrmions in new classes of materials based on combinations of magnetic spin, asymmetric building units with stereo-active lone-pair electrons, and polar lattice symmetry. To demonstrate the viability of the proposed judicious design principles, we successfully synthesized Fe(IO₃)₃ with a non-centrosymmetric polar P6₃ space group, by using low-temperature reactions and characterized the crystal structure using powder synchrotron X-ray diffraction and single-crystal X-ray diffraction. The magnetic structure and magnetic spin evolution of this material were investigated by performing neutron diffraction, ⁵⁷Fe Mössbauer spectroscopy, magnetization, and heat capacity measurements. Density functional theory calculations were also performed to gain better insights into chemical bonding and exchange interaction of Fe(IO₃)₃. Our results demonstrate a connection between topological magnetism and stereo-active lone-pair electrons in compounds having extended polar lattice, expanding avenues to create new skyrmion materials. The results and knowledge gained from this study will be discussed.

This talk starts by reviewing known examples of how topological materials generate new kinds of electrodynamic couplings and effects. We then turn to how topological Weyl and Dirac semimetals can show unique electromagnetic responses, moving from linear optics to nonlinear optics, and the search for specific materials realizing these possibilities. We develop theories for why nonlinear optics is very strong in some Weyl semimetals such as TaAs, and how in others with structural chirality, there is the possibility of a quantized chiral photocurrent. This nonlinear effect has a natural quantum e^3/h^2 and appears in chiral Weyl semimetals over a finite range of frequencies. We discuss interaction and disorder corrections to nonlinear responses in closing, along with a related correction to the anomalous Hall coefficient at finite wavevector.

A quantum anomalous Hall (QAH) state is a two-dimensional topological insulating state that has a quantized Hall resistance of h/(Ce²) and vanishing longitudinal resistance under zero magnetic field (where h is the Planck constant, e is the elementary charge and the Chern number C is an integer). The QAH effect has been realized in magnetic topological insulators and magic-angle twisted bilayer graphene. However, the QAH effect at zero magnetic field has so far been realized only for C = 1. In this talk, I will briefly talk about the route to the QAH effect in magnetically doped TI films/heterostructures. I will focus on our recent progress on the realization of the QAH effect with tunable Chern number (up to C = 5) in multilayer structures consisting of alternating magnetic and undoped topological insulator layers. Moreover, the Chern number of a given multilayer can be tuned by varying either the magnetic doping concentration in the magnetic topological insulator layers or the thickness of the interior magnetic topological insulator layer. In the last part of my talk, I will discuss the potential applications of the QAH insulators with tunable high Chern number and what we can do along this direction in the future.

We acknowledge the support from the DOE grant (DE-SC0019064), the ARO grant (W911NF1810198), the NSF-CAREER award (DMR-1847811), and the Gordon and Betty Moore Foundation’s EPiQS Initiative (Grant GBMF9063 to C.Z.C.).

Discoveries in quantum materials, which are characterized by the strongly quantum-mechanical nature of electrons and atoms, have revealed exotic properties that arise from strong correlations. We anticipate that quantum materials to be discovered and developed in the next years will transform the areas of quantum information processing including communication, storage, computing. To realize this promise, first principles theoretical and computational frameworks for quantum materials are essential. In this context, I will present our recent work in ab initio approaches to the microscopic dynamics, decoherence and optically-excited collective phenomena in quantum matter at finite temperature to quantitatively link predictions with 3D atomic-scale imaging, quantum spectroscopy, and macroscopic behavior. Capturing these dynamics poses unique theoretical and computational challenges. The simultaneous contribution of processes that occur on many time and length-scales have remained elusive for state-of-the-art calculations and model Hamiltonian approaches alike, necessitating the development of new methods in computational physics. I will show our predictions of linear-, nonlinear-, and hydrodynamics in Weyl semimetals, accounting for microscopic electron-electron and electron-phonon scattering processes. I will discuss the anomalous landscape for electron hydrodynamics in systems beyond graphene, highlighting that previously-thought exotic fluid phenomena can exist in both two-dimensional and anisotropic three-dimensional materials with or without breaking time-reversal symmetry. Our work identifies phonon-mediated electron-electron interactions as critical in a microscopic understanding of hydrodynamics. Non-diffusive electron flow, and in particular electron hydrodynamics, has far-reaching implications in quantum materials science, as I will show in this talk. Finally, I will discuss our recent work in driving quantum materials out-of-equilibrium to control the coupled degrees-of-freedom, and present an outlook on similarly controlling newly-synthesized topological systems.

2D materials are an exciting new class of materials that hold great promise for a wide range of engineering applications because of their exceptional tunability. Indeed, these materials’ band gaps, magnetic and topological properties, and conductivity can be dramatically altered just by stretching or stacking them. Despite their promise, however, our modern understanding of 2D materials is partially obscured by the fact that most have only been computationally modeled using Density Functional Theory (DFT), a theory that cannot readily describe electron correlation and is known to yield widely varying results for such central quantities as band gaps and molecular binding energies depending upon the exchange-correlation functional employed.

In this talk, I will describe my group’s recent efforts to model an array of 2D materials using fully correlated quantum Monte Carlo (QMC) techniques and the unanticipated physics these efforts have revealed. First, I will discuss for which materials our simulations reveal the largest discrepancies between DFT and QMC. I will then illustrate the importance of using correlated methods in the modeling of proximity-induced magnetism and spin-orbit coupling in multilayer materials. Lastly, if time permits, I will detail how correlation influences carbon dioxide reduction on post-transition metal chalcogenides. Altogether, this work paints a more complete picture of the electronic structure and properties of this increasingly important class of materials.

High-temperature solutions are promising for discovery of novel materials with interesting properties relevant to superconductivity, magnetism, energy conversion, etc. Despite highly effective for exploratory synthesis, they are much less predictable and offer little to no control of the oxidation state compared to aqueous solutions. Here, we demonstrate that molten hydroxides not only offers crystal growth but also exhibit similar acid-base chemistry like water. Although never before they have been used for the synthesis of a chalcogenide, we found it was surprisingly powerful. By precise oxidation state control in hydroxide mediums, not only we were able to grow single crystals of all known ternary K-Ni-S, we have successfully isolated several new phases. Among them, we have identified a new low-valence nickel-rich sulfide, KNi₄S₂ and discovered polytypism in the kinetically stablized K₂Ni₃S₄. By controlling the polytypism in K₂Ni₃S₄, we could obtain a dilute Kagome lattice, which could be a new spin liquid materials. In addition, using KNi₄S₂ as a template, we obtained a new layered binary Ni₂S by deintercalating K and a LiOH-intercalated Ni₂S by exchanging K with LiOH. This new Van der Waals building block of Ni₂S proves to be a new host layer for intercalation chemistry. The rich acid-base chemistry in molten hydroxides can lead to rational discovery of new materials.

This new compound KNi₄S₂ share great structural similarities to the tetragonal KNi₂S₂. However, unlike KNi₂S₂, it consists of doubly stacked Ni square sheets between two sulfur square sheets instead of edge-sharing NiS₄ tetrahedra. Thus, each Ni forms both Ni-S ionic bonding and Ni-Ni metallic bonding. In addition to its exotic crystal structure, this low-valence Ni compound also shares great similarities with nickelate superconductors. Recently, new excitement emerges following the report of superconductivity (T_c up to 15 K) in a heterostructure of NiO2 infinite-layer on pervoskite.¹ One of the most extraordinary features of this nickelate superconductor is its oxidation state, +1.0-1.2, which is nearly isoelectronic with the holedoped high-T_c cuprate superconductors.² However, unlike the parent phase of the curpate superconductors (antiferromagnetic), no long-range magnetic ordering has yet observed in the nickelate system. Thus, it is of great importance to elucidate the underlying magnetic ordering or lack thereof in the low-valence nickelate system. Here, our new K₂Ni₃S₄ compound not only shares similar Ni-square net with both the nickelate and Fe-based superconductors, but also offers a rare opportunity to study the long-range magnetic ordering in low-valence Ni compounds.

To fully understand their electronic properties, we carried out density functional theory (DFT) calculations. Very interestingly, after deintercalation the binary Ni₂S does not show a simple change in electron-filling. Instead, close to its Fermi level, an electron pocket and almost a hole pocket appear at the G and M points, respectively, whereas there was only an electron pocket at the G point for the parent KNi₄S₂. This feature highly resemble the Fe-based superconductors especially the 122-type KFe₂Se₂ and the LiOH-intercalated FeSe. Although Ni₂S is not superconducting, but the magnetism is suppressed after the deintercalation. In addition, the Fermi surface nesting at the M point is not complete as there is no hole pocket for Ni₂S. However, if we can future oxidize Ni to between Ni +1-1.5, then a electron and hole pocket pair could be created and lead to superconductivity. This will link Fe-based superconductors to the new nickelate superconductors.

1. Li, D.; Lee, K.; Wang, B. Y.; Osada, M.; Crossley, S.; Lee, H. R.; Cui, Y.; Hikita, Y.; Hwang, H. Y. Nature 2019, 572, 624–627.
2. Bednorz, J. G.; Muller, K. A. Zeitschrift fur Physik B Condensed Matter 1986, 64, 189–193.

Single-atom quantum devices, in which the spin state of an atom is used to encode a qubit, are promising architectures for quantum computers. In solid-state Si devices, this requires the precise positioning of subsurface qubit dopant atoms. The current state-of-the-art for single-atom positioning relies on scanning probe techniques such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM). However, these techniques are limited to surfaces and often involve overgrowth layers to encapsulate the dopants, which could lead to the atoms diffusing from their intended positions. Recently, scanning transmission electron microscopy (STEM) has been demonstrated as a tool to directly position single atoms in graphene as well as in silicon by taking advantage of an Ångstrom-size probe that can be manually controlled. The ability to directly locate and position a single dopant atom in a 3D material opens up new avenues of device manufacturing, but the mechanism needs to be well-understood before the technique can be optimized and widely implemented. Here, we study the controlled positioning of subsurface pnictogen dopants in a thin silicon crystal. Using density functional theory molecular dynamics (DFT/MD) simulations, we uncover the atomistic mechanism through which the dopant atoms can be directed, unveiling a novel, damage-free atom exchange process. We study the Si group V dopants Bi, Sb, As, and P, revealing that the atom-exchange mechanism is limited to the large Bi and Sb dopants. New STEM experiments further demonstrate the ability to move the Sb dopants several lattice positions by dragging the electron probe across the sample, confirming that it is possible to manipulate Sb in the same manner that Bi could be directed.

Resistive random access memory (ReRAM) is the emerging nonvolatile memories, which has a metal-oxide-metal structure. A resistive switching (RS) phenomenon offers useful applications for not only nonvolatile memories but machine learning for neural network. The RS phenomenon in a transition metal oxide-based ReRAM cell is usually explained as nonvolatile transitions in the cell resistance between high- and low- resistance states by formation and dissolution of localized conductive filaments in the oxide thin film, which is created by the first voltage application, called forming. I reported that conductance quantization in nickel-oxide (NiO)-based ReRAM cells sandwiched by platinum (Pt) electrodes was observed during forming [1,2]. When the scale of the weakest spot in the filament reaches an atomic size, conductance quantization can be expected to appear. Therefore, the RS cell includes a conductive filament with a quantum point contact (QPC) upon the voltage sweep [1]. However, variation of voltage to reach the quantized conductance was large, because the amount controllability of oxygen vacancies was possibly inadequate. In this study, I investigate hydrogen thermal treatment effects on how the amount or distribution of the oxygen vacancies are controlled.

A 60-nm-thick Pt bottom electrode was deposited by sputtering on a silicon-dioxide/p-silicon substrate. Next, a polycrystalline 80-nm-thick NiO with a columnar structure layer was deposited by radio-frequency reactive sputtering under oxygen gas flow rate precisely regulated. Pt top electrodes were subsequently deposited on the NiO layer. Several Pt/NiO/Pt ReRAM cells showed the first abrupt current jump by an initial voltage sweep to the cell, which made the cell conductance equivalent to the quantized conductance (=2e²/h). Moreover, the continued voltage sweep brought about conductance quantization in the cells, indicating the formation of a conductive filament with a QPC. However, variation of the voltage to start the current jump was large and only several cells showed conductance quantization.


I attempted various thermal treatment of the RS cells in reduced or oxidized gas ambience. Thermal treatment above 300°C during more than 20 min resulted in poor RS endurance, indicating deterioration of ReRAM cells by a strong heat effect. Meanwhile, argon and oxygen thermal treatment below 200°C led to a subtle change of initial cell resistance (less than 5%) and few effects on the voltage variation and how many cells showed clear conductance quantization. Therefore, thermal treatment below 200°C was conducted in hydrogen gas, which possesses a high reducing reaction, to prevent the cells from heat deterioration effect.

Although hydrogen thermal treatment between 150°C and 200°C resulted in the delamination of Pt top electrodes during even less than 10 min, initial resistance of cells fabricated with a higher oxygen gas flow rate during NiO deposition decreased to moderate values and RS endurance in the treated cells was almost improved by the hydrogen treatment below 150°C. In high-angle annular dark field scanning transmission electron microscopy analyses of the NiO layers after the hydrogen thermal treatment, brightness of bright spots observed at grain-boundary triple-points in the NiO layers was similar with each other. Energy dispersive X-ray spectroscopy revealed that the average composition ratio of O to Ni along the thickness direction at triple-points was comparatively constant and smaller than the value within grains. These results indicate that conductive filaments with a QPC are formed along Ni- or oxygen-vacancy-rich grain-boundary triple-points, and that reducing reaction by hydrogen thermal treatment below 150°C leads to restriction of voltage variation and improvement of appearance probability of conductance quantization.

[1] Y. Nishi et al., J. Mater. Res. 32, 2631 (2017).
[2] Y. Nishi et al., J. Appl. Phys. 124, 152134 (2018).

Exciton-polaritons are a hybrid state of light and matter and are a quantum playground for exploring Bose-Einstein condensation, superfluidity, quantum vortices, and quantum entanglement. Although room-temperature polariton condensation has now been observed in carbon nanotubes, organic molecules, and in lower dimensional metal halide perovskites, these coherent quantum states are difficult to control and study due to rapid photodegradation. Here, we show orders-of-magnitude improvements in the photostabilization of the lower polariton branch emission from J-aggregated cyanine dyes through use of amine-based light stabilizers and hygroscopic crystal matrices. These results will likely enable condensation in a wide range of materials where these quantum effects were previously inaccessible.